Technologies for estimating aging level or discharge ability of a battery installed in a motor vehicle and the like are known since the past and there is one as described in Patent Document 1 for example.
Because impedance of a battery is strongly correlated with aging level or discharge ability of the battery in general, it is possible to estimate the aging level or the discharge ability of the battery if the impedance of the battery can be detected and is used. It then becomes possible to determine whether the battery is depleted or the discharge ability is lowered by estimating the aging level or the discharge ability of the battery and to urge a user of the battery to replace the battery when the battery is depleted or the discharge ability is lowered.
As a method for estimating the aging level or the discharge ability of a battery in a power system provided with the battery, there has been known a method of passing a predetermined charge current or discharge current to the battery, of measuring the current and voltage at that time and of calculating impedance from the measured current and voltage through predetermined arithmetic operations.
While the impedance of the battery becomes an index that accurately indicates the aging level or the discharge ability of the battery in a well-conditioned environment such as a laboratory, the measurement of the impedance is carried out actually in environments exposed to noises generated from electrical equipments typically in measuring impedance of a car battery for example. Due to that, the battery aging determining method described in Patent Document 1 proposes a method of passing a discharge current having a constant frequency by discharging at a constant period and by Fourier-transforming current and voltage waveforms of the discharge current to obtain the impedance.
[Patent Document 1] Japanese Patent No. 3367320